Temporal Velocity Research Articles

Test of the relation between temporal and spatial [formula omitted] by Knopoff et al.

The quality factor is a dimensionless measure of the energy loss per cycle of wave modes in an attenuation medium. Accurate measurement is important in various fields, from seismological studies to detect zones of partial melting to the geophysics of reservoirs to study rock properties such as porosity, fluid properties and saturation and permeability. In seismology, the quality factors measured for normal (standing) modes and propagating waves differ, as well those of equivalent experiments based on resonant rods and ultrasonic pulses performed in the laboratory. These measurements result in temporal and spatial quality factors respectively. A relationship between these two different quality factors and between the corresponding attenuation factors was proposed by Knopoff et al. sixty years ago. The conversion factor is basically the ratio between the phase velocity and the group velocity, while for the attenuation factor is the group velocity. We test these relations, which hold for low-loss solids, for body waves, using a Kelvin-Voigt rheology and a constant Q model, which provide explicit expressions of the temporal and spatial quality factors and velocities involved in these relations. The proposed theory provides the basis for a complete characterization of temporal and spatial quality factors and velocity dispersion based on arbitrary stress–strain relationships.

Harmonic chain driven by active Rubin bath: transport properties and steady-state correlations

Characterizing the properties of an extended system driven by active reservoirs is a question of increasing importance. Here, we address this question in two steps. We start by investigating the dynamics of a probe particle connected to an ‘active Rubin bath’: a linear chain of overdamped run-and-tumble particles. We derive exact analytical expressions for the effective noise and dissipation kernels, acting on the probe and show that the active nature of the bath leads to a modified fluctuation–dissipation relation. In the next step, we study the properties of an activity-driven system, modelled by a chain of harmonic oscillators connected to two such active reservoirs at the two ends. We show that the system reaches a non-equilibrium stationary state (NESS), remarkably different from that generated due to a thermal gradient. We characterize this NESS by computing the kinetic temperature profile, spatial and temporal velocity correlations of the oscillators, and the average energy current flowing through the system. It turns out that the activity drive leads to the emergence of two characteristic length-scales, proportional to the activities of the reservoirs. Strong signatures of activity are also manifest in the anomalous short-time decay of the velocity autocorrelations. Finally, we find that the energy current shows a non-monotonic dependence on the activity drive and reversal in direction, corroborating previous findings.

Space‐Time Monitoring of Seafloor Velocity Changes Using Seismic Ambient Noise

AbstractWe use seismic ambient noise recorded by dense ocean bottom nodes (OBNs) in the Gorgon gas field, Western Australia, to compute time‐lapse seafloor models of shear‐wave velocity. The extracted hourly cross‐correlation (CC) functions in the frequency band 0.1–1 Hz contain mainly Scholte waves with very high signal‐to‐noise ratio. We observe temporal velocity variations (dv/v) at the order of 0.1% with a peak velocity change of 0.8% averaged from all station pairs, from the conventional time‐lapse analysis with the assumption of a spatially homogeneous dv/v. With a high‐resolution reference (baseline) model from full waveform inversion of Scholte waves, we present an elastic wave equation based double‐difference inversion (EW‐DD) method, using arrival time differences between the reference and time‐lapsed Scholte waves, for mapping temporally varying dv/v in the heterogeneous subsurface. The time‐lapse velocity models reveal increasing/decreasing patterns of shear‐wave velocity in agreement with those from the conventional analysis. The velocity variation exhibits a ∼24‐hr cycling pattern, which appears to be inversely correlated with the diurnal variations in sea level height, possibly associated with dilatant effects for porous, low‐velocity shallow seafloor and rising pore pressure with higher sea level. This study demonstrates the feasibility of using dense passive seismic surveys and wave‐equation time‐lapse inversion for quantitative monitoring of subsurface property changes in the horizontal and depth domain.

Wake structure of an array of cylinders in shallow flow

Although there is a range of approaches for classifying the wake structure behind an array of obstacles, these approaches provide inconsistent results across different array systems. This motivates the present study to integrate and reconcile these approaches into one that is consistent across different systems. This new, transferable classification approach is based on the dimensionless flow blockage of the array and the wake stability parameter. To demonstrate this approach, a series of laboratory experiments was conducted to characterise the wake structure behind an array of emergent cylinders across a practically relevant parameter space that has not previously been explored. Two arrays with the same values of flow blockage and wake stability parameters but different sizes display the same wake structure, demonstrating the controlling influence of these two parameters on the wake structure. This approach classifies four different wake structures, which are distinct in that they display differences in instantaneous and time-averaged flow fields, temporal velocity oscillations, shear layer growth and the length of the steady wake region. The dependence of the wake structure on the two parameters is a consequence of (i) the controlling influence of blockage on the fraction of incident flow passing through the array and (ii) the ability of shallowness to suppress wake instabilities and, to a lesser extent, also influence the velocity through the array. This paper provides a predictive framework for the wake structure based on knowledge of the array geometry, and the depth and velocity of incident flow across the entire relevant practical parameter space.

Temporal Seismic Velocity Changes Associated With the Mw 6.1, May 2008 Ölfus Doublet, South Iceland: A Joint Interpretation From dv/v and GPS

AbstractIn South Iceland, populated and agricultural areas are at risk of earthquakes due to their location within the South Iceland Seismic Zone (SISZ). In 2008, two moderate‐sized earthquakes (M5.8 and M5.9) occurred in Ölfus, the western end of this highly active transform zone. We analyze temporal seismic velocity variations (dv/v) related to the Ölfus earthquake doublet, using cross‐correlations of ambient noise in the frequency range of 0.1–3.0 Hz. The two mainshocks decrease the average velocity by 0.8% at the nearest stations. The co‐seismic changes are most noticeable from 0.7 to 1.7 Hz and affect a 40 km wide region. We present a first‐time comparison of dv/v to crustal deformation, seismicity, co‐seismic volumetric stress changes and reported PGA distribution for the Ölfus doublet. Ground accelerations caused by mainshocks at intermediate distances suggest that strong shaking‐related damage may contribute to the co‐seismic dv/v decrease. A rapid velocity increase (0.3%) in a month after the co‐seismic drop indicates crustal rock healing. We find 3‐months of post‐seismic decorrelation, followed by a nearly permanent velocity decrease (0.2%) confined to a shallow layer (1 km) until the end of the observation period. Afterslip and pore fluid effects in the near‐source region are likely to influence post‐seismic dv/v. We demonstrate that seismic interferometry can contribute to future fault‐zone monitoring operations in the SISZ by detecting small changes in velocity.

Performance evaluation of cyclone separators with elliptical cross-section using large-eddy simulation

In the present study, cyclone separators with elliptical cross-sections have been studied numerically. Five cyclone models are considered here with the normalized (semi-) major axis (a) and minor axis (b) as: (a/D, b/D) = (0.51, 0.49), (0.52, 0.48), (0.53, 0.47), (0.54, 0.46), and (0.55, 0.45), referred to as the models A, B, C, D, and E, respectively. The standard model is of circular cross-section with a body diameter, D = 0.205 m, and is the reference model for performance comparison. It has been ensured that all the cyclones bear a similar hydraulic diameter, and hence, a nearly similar volume. Apart from the pressure drop and collection efficiency, we also evaluate the periodicity associated with the precessing vortex core around the cyclone axis by performing a fast Fourier transformation to the temporal velocity signal. Conclusive results indicate that both the pressure losses as well as collection efficiencies reduce with increasing the major axis (and reducing the minor axis) values. The mean velocity components undergo significant changes than the velocity fluctuations with stretching of the elliptical cross-section. The models with the largest major axis (or smaller minor axis) may be used as pre-separators to remove larger particles at the lowest pressure drop values.

The impact of operating temperatures on the fluctuating flow field and precessing vortex core in cyclone separator using large-eddy simulations

The present study evaluates numerically the impact of the operating temperature of gas on the cyclone performance viz. the pressure drop, collection efficiency, and flow field details at an inlet velocity, Uin=15 m/s. The gas temperature in a range of 273–1073 K is considered to significantly vary the fluid density and viscosity. For an in-depth analysis, we use advanced closure large-eddy simulation (LES) with the standard Smagorinsky model for treating the unresolved scales. LES can accurately provide additional details on the precessing vortex core phenomena that give rise to enhanced fluctuations in the core region of the cyclone. Apart from the traditional fast Fourier transformation analysis to evaluate the periodicity in the signal, we also perform continuous wavelet transformation and empirical mode decomposition operation on the temporal velocity signals for a better understanding of the flow instabilities—the signals reveal variations of frequency components with time, indicating a non-stationary behavior. It has been observed that an increase in the gas temperature causes lateral contraction of the inner vortex followed by the reduction in its precessional frequency about the cyclone axis with a significantly increased level of noise in the spectra. Furthermore, both pressure losses and collection efficiency largely reduce due to the weakening of swirling strength and enhancement in the fluctuating velocity components with an increase in the gas temperature.

Numerical simulation of wall shear stress and boundary layer flow from jetting cavitation bubble on unheated and heated surfaces

The physics of wall shear stress and boundary layer flow from jetting cavitation bubbles on unheated and heated surfaces was investigated numerically using a homogeneous mixture model based on a fully compressible Navier–Stokes solver. A spatial–temporal shear stress map and velocity field on unheated surfaces under several standoffs were considered first. Complex boundary layer flow, including separation vortex caused by an adverse pressure gradient and the generation of a thermal boundary layer because of skin friction with the shear flow, was discussed in detail. The tendency of different maximum shear stress for each standoff section was discussed with liquid film thickness and jet velocity. Also, we suggested maximum shear stress correlation equation for minimum film thickness and wall impact pressure as dimensionless form. As an expansion of previous studies, we examined bubble collapse–induced wall shear stress on a heated surface under various surface temperatures. The peak shear stress decreased with the lower Prandtl number flow. In the collapse phase, surface cooling by downward microjet and heat transfer with ring vortex were also captured. Consequently, we suggested correlation equations for primary and secondary peak shear stresses as function of Prandtl number and confirmed that the boundary layer flow including heat transfer can be well simulated.

Elutriation of fine particles in a pulsed fluidized bed

This paper reported on fine particle elutriation for two different static bed heights of 25 cm and 30 cm in a pulsed fluidized bed. The effects of three types of flow including continuous airflow, on-off pulsed airflow, and pulsed airflow imposed on continuous airflow were investigated. Observations showed that the elutriation fraction extremely increased with gas temporal average velocity and static bed height for continuous airflow, while for pulsed and mixed flows at the same condition the increase was not considerable. The effect of pulse frequency from 0.033 Hz to 0.1 Hz as an optimum range led to an extreme reduction loss of particles for pulsed and mixed airflows compared to continuous airflow even at high gas temporal average velocities. The elutriation rate of mixed airflow at 0.033 Hz and 3Umf for 30 cm static bed height on average was 75% and 35.3% less than continuous and pulsed airflows, respectively.

Accelerating melting time in a triplex tube heat exchanger thermal energy storage unit with micro-channels

This article studies optimal conditions to maximize heat transfer efficiency within a triplex tube heat exchanger (TTHX) filled with phase-change material. In this context, micro-channels (MCs) incorporated into TTHX for the first time for acceleration melting time. A two-dimensional numerical model has been developed, and pure conduction and natural convection are both simulated. The Reynolds numbers range from 1 to 100. The types of temporal velocity profiles into micro-channels were uniform, step, elliptic, sinusoidal, and power. The effect of gravity states, namely no gravity, with gravity, and (MCHE), has been studied with different numbers of microchannels (4, 6, 8, 10, 12, and 14). The results showed that there was no variation in melting duration observed when increasing the Re number from 50 to 100. Thus, the maximum reduction in melting period occurred at Re = 50. Moreover, sinusoidal velocity profiles were more effective and could help the process finish sooner. In addition, micro channel tube exchanger (MCTE) fitted or equipped with 10 micro-channels has an optimum condition considering pressure drop and complete melting time. Using the sinusoidal profile reduced the total fusion time by 325 s compared to uniform flow (8.47 % reduction). This indicates that the sinusoidal profile is an effective way to reduce the melting period.

Experimental and numerical analyses of gravity-driven granular flows between vertical parallel plates for solar thermal energy storage and transport

Gravity-driven granular flows between vertical parallel plates were considered at elevated temperatures to examine flow behavior and inform solar thermal energy storage and transport infrastructure design and performance. A series of experiments was performed at inlet temperatures of 23, 200, 400, 600, and 800 °C in a high-temperature granular flow experimental rig and spatial and temporal velocity and surface temperature fields were measured. Granular flows were observed to change as a function of temperature. The interaction between the particles and the wall significantly increased with decreases in channel widths. Particle velocities at the outlet of the channel were compared with results from numerical model using the discrete element method. Temperature-dependent flow properties directly related to the assembly were measured and used for granular flow simulation. Flow behavior within the channel was investigated from the simulated results by obtaining particle volume fractions and particle velocities. Wall temperature and outlet granular flow temperature were monitored during the flow experiment and temporal heat loss in the flow was calculated. The granular flow behavior and heat transfer analysis from both the experiment work and simulation provide important mass transfer information for heat transfer modeling at elevated temperature.

Tracking model predictive control and moving horizon estimation design of distributed parameter pipeline systems

This manuscript proposes moving horizon control and state/parameter estimation designs for pipeline networks modelled by partial differential equations (PDEs) with boundary actuation. The spatial–temporal pressure and velocity dynamics within the pipelines are described by a system of six coupled one-dimensional first-order nonlinear hyperbolic PDEs. To address the discrete-time modelling challenge and preserve the infinite-dimensional nature of the pipeline system, the Cayley–Tustin transformation is deployed for model time discretization without any spatial discretization or model reduction. Considering the lack of full state information across the entire pipeline manifold, unknown states and uncertain parameters are estimated using moving horizon estimation (MHE). Based on the estimated states and parameters, a tracking model predictive control (MPC) strategy for the discrete-time infinite-dimensional pipeline system is proposed, which enables specific operation while ensuring physical constraint satisfaction. The effectiveness of the proposed controller and estimator designs is demonstrated via numerical examples.

Melting numerical simulation of hydrated salt phase change material in thermal management of cylindrical battery cells using enthalpy-porosity model

Battery thermal management using different cooling techniques is rapidly growing. Understanding the proper cooling and melting process when phase change materials (PCM) are used is of prime importance in this area. Hence, a transient thermal-fluid and melting process of hydrated salt PCM enclosed in a battery module with six cylindrical cells is numerically investigated to understand the melting process of the PCM. Four structural models S1, S2, S3, and S4 are constructed for the present numerical simulation. The battery cell wall is kept at a constant temperature of 35℃, while the rectangular enclosure walls are assumed to be insulated. A finite volume scheme-based CFD (computational fluid dynamics) software is used to simulate the melting process of hydrated salt PCM. In order to capture the phase change phenomenon from solid to liquid, an enthalpy-porosity equation is solved. The temporal temperature distribution, liquid fraction, velocity and enthalpy are analyzed. The results obtained by the numerical computation suggest that the battery cell arrangement used in S1 and S2 model at the initial time step gives better space for temperature distribution and liquid fraction up to the time step of 420 s, while S3 and S4 model after a time interval of 420 s provide better scope for temperature distribution and complete melting of hydrated PCM.

Peculiarities of radial velocity variability of lines in the spectrum of α Cyg supergiant

ABSTRACT We use about 430 CCD spectra obtained in 1998–2001 and in 2007 to study temporal radial velocity variations of the ion, He i, and H β lines in the spectrum of the α Cyg. We found that these variations are caused by pulsation-type motions. In case of the ion and He i lines, fluctuations occur in the fundamental radial mode with the period of about 12.5 ± 1.0 d and the amplitude of 5.0 ± 0.5 km s−1. The oscillation lasts about ∼34 ± 1 d. In the case of the H β line the radial velocity variability was studied separately for the blue and red halves of the line profile. Such a methodical approach allowed to discover three characteristic cases of radial velocity variability for the blue and red halves of the H β line profile. In 1999 and 2001, the period P and amplitude A variability parameters are the same for the blue and red halves, and hence for the line bisector too. In 1998, P and A are the same for the blue and red halves of the profile, however, there is a phase shift between the fluctuations of their radial velocity by 0.6P. In 2000, P and A are different for the two halves of the line profile. Relative to the center of the star’s mass, the average expansion rate for the atmospheric layers, where the H β line is formed, is about −10.0 km s−1. It varies over time with a semi-amplitude of around ±3.3 km s−1.

Heavy inertial particles in rotating turbulence: Distribution of particles in flow and evolution of Lagrangian trajectories.

We revisit the problem of heavy particles suspended in homogeneous box turbulence flow subjected to rotation along the vertical axis, which introduces anisotropy along the vertical and horizontal planes. We investigate the effects of the emergent structures due to rotation, on the spatial distribution and temporal statistics of the particles. The distribution of particles in the flow are studied using the joint probability distribution function (JPDFs) of the second and third principle invariants of the velocity gradient tensor, Q and R. At high rotation rates, the JPDFs of Lagrangian Q-R plots show remarkable deviations from the well-known teardrop shape. The cumulative probability distribution functions for times during which a particle remains in vortical or straining regions show exponentially decaying tails except for the deviations at the highest rotation rate. The average residence times of the particles in vortical and straining regions are also affected considerably due to the addition of rotation. Furthermore, we compute the temporal velocity autocorrelation and connect it to the Lagrangian anisotropy in presence of rotation. The spatial and temporal statistics of the particles are determined by a complex competition between the rotation rate and inertia of the particle.

Thermo-fluid performance enhancement in a long double-tube latent heat thermal energy storage system

Thermodynamic analysis and flow rate optimization for the long double-tube latent heat thermal energy storage systems (LHTESS) are performed. Computer modeling is carried out using created software and is based on a developed 3D non-steady non-linear coupled thermo-fluid mathematical model that combines the apparent heat capacity method and finite volume method for the sodium nitrate (NaNO3) phase change materials (PCM) and for the Syltherm800 heat transfer fluid (HTF). The mathematical model and numerical algorithm are verified through comparison with experimental data. It is found that the laminar flow of HTF ensures the uniform temperature in PCM and in HTF and this temperature is equal to the PCM melting temperature. It is proved that the PCM and HTF average temperatures along the LHTESS can be equalized using spatial and temporal velocity variations of the HTF. Mutual impact of the heat transfer and fluid mechanics parameters in the long double-tube LHTESS was studied by correlation between Nusselt numbers and Reynolds numbers. The optimal HTF flow rate parameters were analyzed based on the non-equilibrium thermodynamics method. It is discovered that the laminar regime 0 < Re < 2000 and the turbulent regime 12000<Re<15000 are the most optimal for long double-tube LHTESS of the described geometry.

Temporal Prediction of Landslide-Generated Waves Using a Theoretical–Statistical Combined Method

For the prediction of landslide-generated waves, previous studies have developed numerous empirical equations to express the maximums of wave characteristics as functions of slide parameters upon impact. In this study, we built the temporal relationship between the wave characteristics and slide features. We gave specific insights into impulse waves generated by snow avalanches and mimicked them using a buoyant material called Carbopol whose density is close to that of water. Using the particle image velocimetry (PIV) technique, the slide’s temporal velocity field and thickness, as well as the temporal free water surface fluctuation, were determined experimentally. Using a statistical method denoted as panel data analysis, we quantified the temporal wave amplitude from the time series data of the thickness and depth-averaged velocity of the sliding mass at the shoreline. Then, the slide’s temporal thickness and velocity at the shoreline were estimated from the parameters of the stationary slide at the initial position, based on the viscoplastic theory. Combining the panel data analysis and the viscoplastic theory, the temporal wave amplitudes were estimated from the initial slide parameters. In the end, we validated the proposed theoretical–statistical combined predictive method with the support of experimental data.

Flow kinematics of granular materials considering realistic morphology

The study focuses on understanding the influence of particle morphology, interparticle friction and hopper exit width on spatial and temporal variations in velocity during dry granular flow. Using non-convex morphologies (sand, chickpea and rice), the flow characteristics and velocity fluctuations are explored. Funnel flow pattern was evident in angular shapes even in hoppers designed for mass flow discharge. K-means clustering, an unsupervised algorithm, is used to group the spatial variation of velocity inside the hopper. Three flow clusters were identified which indicate the flow transition from funnel to mass flow. The Free Flow Cluster (FFC) height was observed to be least for bulky-shaped particles because of their high sphericity and roundness values. Angular particles were observed to exhibit rapid temporal velocity fluctuations when compared to elongated and bulky shapes. Finally, the pseudo-shape approximations typically used in DEM fail to capture the true interlocking nature in the flow behaviour observed within realistic particle morphologies.

The in vivo measurement of replication fork velocity and pausing by lag-time analysis

An important step towards understanding the mechanistic basis of the central dogma is the quantitative characterization of the dynamics of nucleic-acid-bound molecular motors in the context of the living cell. To capture these dynamics, we develop lag-time analysis, a method for measuring in vivo dynamics. Using this approach, we provide quantitative locus-specific measurements of fork velocity, in units of kilobases per second, as well as replisome pause durations, some with the precision of seconds. The measured fork velocity is observed to be both locus and time dependent, even in wild-type cells. In this work, we quantitatively characterize known phenomena, detect brief, locus-specific pauses at ribosomal DNA loci in wild-type cells, and observe temporal fork velocity oscillations in three highly-divergent bacterial species.

Stratification of Children with Autism Spectrum Disorder Through Fusion of Temporal Information in Eye-gaze Scan-Paths

Background: Looking pattern differences are shown to separate individuals with Autism Spectrum Disorder (ASD) and Typically Developing (TD) controls. Recent studies have shown that, in children with ASD, these patterns change with intellectual and social impairments, suggesting that patterns of social attention provide indices of clinically meaningful variation in ASD. Method: We conducted a naturalistic study of children with ASD (n = 55) and typical development (TD, n = 32). A battery of eye-tracking video stimuli was used in the study, including Activity Monitoring (AM), Social Referencing (SR), Theory of Mind (ToM), and Dyadic Bid (DB) tasks. This work reports on the feasibility of spatial and spatiotemporal scanpaths generated from eye-gaze patterns of these paradigms in stratifying ASD and TD groups. Algorithm: This article presents an approach for automatically identifying clinically meaningful information contained within the raw eye-tracking data of children with ASD and TD. The proposed mechanism utilizes combinations of eye-gaze scan-paths (spatial information), fused with temporal information and pupil velocity data and Convolutional Neural Network (CNN) for stratification of diagnosis (ASD or TD). Results: Spatial eye-gaze representations in the form of scanpaths in stratifying ASD and TD (ASD vs. TD: DNN: 74.4%) are feasible. These spatial eye-gaze features, e.g., scan-paths, are shown to be sensitive to factors mediating heterogeneity in ASD: age (ASD: 2–4 y/old vs. 10–17 y/old CNN: 80.5%), gender (Male vs. Female ASD: DNN: 78.0%) and the mixture of age and gender (5–9 y/old Male vs. 5–9 y/old Female ASD: DNN:98.8%). Limiting scan-path representations temporally increased variance in stratification performance, attesting to the importance of the temporal dimension of eye-gaze data. Spatio-Temporal scan-paths that incorporate velocity of eye movement in their images of eye-gaze are shown to outperform other feature representation methods achieving classification accuracy of 80.25%. Conclusion: The results indicate the feasibility of scan-path images to stratify ASD and TD diagnosis in children of varying ages and gender. Infusion of temporal information and velocity data improves the classification performance of our deep learning models. Such novel velocity fused spatio-temporal scan-path features are shown to be able to capture eye gaze patterns that reflect age, gender, and the mixed effect of age and gender, factors that are associated with heterogeneity in ASD and difficulty in identifying robust biomarkers for ASD.